A method for hydrorefining crude oil by mixing the crude oil with hydrogen, heating and bringing into contact with hydrorefining catalysts loaded in multiple catalyst layers is known as a typical hydrorefining method for obtaining low-sulfur gas oil with a sulfur content of 500 ppm or less. In this method, there are also cases wherein hydrogen is further introduced between the multiple catalyst layers. However, the concentration of impurities, such as hydrogen sulfide, ammonia, etc., produced by hydrorefining contained in the hydrorefined hydrocarbons near the outlet of the catalyst layers rises and therefore, hydrorefining wherein contents of the sulfur and nitrogen are adequately reduced is difficult. The hydrogen sulfide, ammonia, etc., inhibit catalytic reaction and as a result, activity of the catalyst is markedly reduced. Consequently, as disclosed in U.S. Pat. Nos. 5,705,052 and 5,720,872, hydrogen sulfide and ammonia gas that have been dissolved in the hydrorefined hydrocarbons are stripped inside a vessel separate from the reaction column in which the catalyst layers have been set up.
With this type of hydrorefining unit, hydrogen and feed oil are brought into contact with catalyst as a cocurrent. The method is known whereby the hydrogen sulfide and ammonia impurities dissolved in the effluent are removed by allowing hydrogen to flow into the catalyst as a countercurrent to the liquid hydrocarbons (feed oil) that flow through the catalyst, as described in, for instance, Japanese Patent No. 2,617,158.
However, it is difficult to hydrorefine gas oil distillates in order to bring the sulfur content to 150 ppm or less, particularly 50 ppm or less, with the above-mentioned conventional unit and method. In particular, there is a problem that the allowable range of the gas quantity of flow and feed oil quantity of flow with which stable operation is possible is narrow in hydrorefining units with which the previously mentioned hydrogen and feed oil are brought into contact with catalyst as cocurrent and therefore, operation is not simple.